Becker's Muscular Dystrophy

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Becker's Muscular Dystrophy

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The muscular dystrophies (MDs) are a group of inherited disorders characterised by progressive muscle wasting and weakness. Becker's muscular dystrophy (BMD) is similar to the more common muscular dystrophy - Duchenne muscular dystrophy (DMD) - but the clinical course is milder. As with DMD, there is muscle wasting and weakness which is mainly proximal. Generally, walking difficulties begin after the age of 16.[1]

Female carriers of BMD may be affected, either by some degree of muscle weakness and/or by cardiomyopathy.

Epidemiology

The incidence is about 1 in 17,000 live births (about one fifth of DMD incidence).[2] BMD is caused by abnormalities of the dystrophin gene, which is responsible for the muscle protein dystrophin. In BMD, abnormal but only partly functional dystrophin is produced (cf DMD, where dystrophin is lacking).

It is inherited in an X-linked recessive pattern. There are various gene defects which give rise to BMD, affecting different parts of the dystrophin gene. The clinical severity of BMD varies and this is probably due to:

Variation in the individual genetic defects and hence in the dystrophins thus produced.

Factors other than the dystrophin gene, which determine the severity of BMD in an individual. In one reported family, two brothers with an identical gene defect had very different clinical manifestations of BMD.[1]

BMD can occur as a new mutation. Therefore, not all mothers of BMD patients will be carriers of the gene. BMD can also occur through mosaicism (where only some cell lines are affected).

Symptoms

Symptoms usually begin in childhood. The average age at diagnosis is 11 years but there is a wide age range. The clinical severity varies.

Early symptoms

Delayed walking (sometimes).

Muscle cramps on exercise.

Most BMD children are not 'athletic' and may struggle with school sports.

Later symptoms

Muscle weakness:

Affects the proximal muscles of the limbs mainly.

May begin in teenage years or 20s, causing difficulty in climbing stairs, fast walking and lifting heavy objects.

BMD patients can walk independently until the age of 16 or later (cf DMD, where patients cannot walk beyond the age of 12). Walking ability is lost (usually at the age of 40-60) but sometimes earlier, around the age of 20-30.

Signs

Wasting of the proximal muscles; hypertrophy of others, particularly the calf muscles.

Musculoskeletal complications

The severity of cardiomyopathy and congestive heart failure may not parallel the severity of skeletal muscle disease. Atrial and ventricular arrhythmias may be life-threatening. The degree of hypoventilation and pulmonary dysfunction also affects cardiac function in muscular dystrophy.[5]

Careful assessment and monitoring are required with any anaesthetic procedure or surgery.

Other complications

Respiratory muscle weakness:

Depending on the clinical severity of muscle weakness, this is a possible complication. Management would be similar to that for DMD respiratory complications. See the separate Duchenne Muscular Dystrophy article.

Most carriers are asymptomatic but a small percentage (2-5%) may have skeletal muscle symptoms; they are known as manifesting carriers of BMD:

The reason why the gene manifests in some women but not in others may be through the mechanism of 'X-inactivation', where the normal X chromosome is inactive and the X chromosome carrying the BMD mutation is the active one.

As with BMD boys, there may be no family history of the disease.

Some cases of BMD manifesting carriers were previously diagnosed as having another type of muscular dystrophy but, with new techniques such as dystrophin staining, have been identified as having BMD.

Clinical features

There is wide individual variation in the severity of symptoms - from mild muscle weakness, aches or calf muscle enlargement, to a disease as severe as that in boys.

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